1. Field of the Invention
The present invention relates to an optical recording medium which is capable of high-density recording and reproduction, and a recording/reproducing method therefor.
2. Description of the Background Art
In recent years, the so-called MSR (magnetically induced super resolution) system is watched with interest as super resolution means for recording/reproducing information in/from a region smaller than a recording spot which is decided by a light diffraction limit. Examples of such an MSR system are disclosed in Technical Digest of Optical Data Storage Topical Meeting 1991, Vol. 5, pp. 112 to 115 (Lecture No. TuB-3) and pp. 116 to 119 (Lecture No. TuB-4), for example. This system is characterized in that a magneto-optical recording film consisting of a plurality of layers is employed with provision of a masking layer for masking peripheral information other than that of an information recording layer. Recorded information (recorded marks) is transferred to the masking layer which is increased in temperature by irradiation with a relatively strong reproducing beam, whereby influences by adjacent tracks and recorded marks in a linear density direction are suppressed and the optical resolution is improved even if the recording density is increased.
As to an optical recording medium for attaining a similar super resolution effect, Japanese Patent Laying-Open No. 5-225611 (1993) discloses a medium which is provided with a layer including a light absorption center causing nonlinear light absorption such as saturable absorption as a masking layer, for example. The saturable absorption is such a property that electrons are efficiently excited when the beam intensity is weak since most of electrons in the absorption center are in ground states while the ratio of electrons excited from ground states is reduced when the beam intensity is strong since most of electrons are excited. Thus, this layer absorbs light when the light intensity is low, while its absorbance is reduced when the light intensity is increased. When the medium which is provided with the layer having such a saturable absorption property is irradiated with a strong reproducing beam for reproducing information, therefore, the reproducing beam will not reach the recording layer in the periphery of its spot due to high absorption for low light intensity, while the reproducing beam can reach the recording layer around the center of the spot due to small absorption for high light intensity. Thus, it is possible to attain a super resolution effect similarly to the aforementioned MSR system.
Japanese Patent Laying-Open No. 5-242524 (1993) also discloses a recording/reproducing method employing an additive layer which contains spiro selenazolino benzopyran exhibiting reverse photochromism and whose transmittance changes in response to the intensity of irradiation light. As to a similar technique, further, Japanese Patent Laying-Open No. 5-266478 (1993) discloses a method employing a masking layer, which is non-transmittable with respect to a reproducing beam in general, having such a property that only its central portion exhibits transmissibility upon irradiation with a reproducing beam which is so controlled that its central portion is in excess of a prescribed intensity level and again exhibits non-transmissibility after passage of the reproducing beam. The material for this masking layer may be prepared from indoline spiropyran exhibiting reverse photochromism.
In the conventional method employing the aforementioned MSR system, however, the recording medium is disadvantageously restricted to a magneto-optical recording medium. Further, it is difficult to attain a high C/N ratio since the temperature of a magnetic layer provided in the medium is increased through a beam of relatively high intensity for reproduction. When as studied in general a green or blue laser beam is employed so that a recording/reproducing laser beam is reduced in wavelength for high densification, it is necessary to employ a new magneto-optical recording material which is different from those for the near infrared laser beam as currently employed. In this case, it is necessary to design new magnetic properties of the magnetic layer, in order to implement the MSR effect. Further, an influence is exerted by thermal diffusion due to an effect of heat which is employed for transferring recorded information to the masking layer, and hence it is difficult to apply the method to mark edge recording, which is effective means for high densification.
In the method of employing the layer including the absorption center having nonlinear light absorption as the masking layer, the recording medium is not restricted to a magneto-optical recording medium, dissimilarly to the MSR system. In this method, however, extremely high light intensity is generally required in order to cause saturable absorption, leading to such a new problem that information recorded in the recording layer may be destroyed by heat. While information must be recorded at larger light intensity than that in reproduction, recorded marks may be spread by an influence of thermal diffusion to disadvantageously reduce the recording density. When a material causing a reverse photochromic reaction is employed, the masking layer reacts by heat such that a coloring reaction of the mask layer may be simultaneously caused in reproduction by the heat reaction, leading to imperfect improvement in transmittance of the mask layer and reduction of the super resolution effect. Further, a bad influence may be exerted by the aforementioned thermal diffusion while recording efficiency may be reduced due to absorption of recording beam energy by the masking layer in recording. Further, it is impossible to attain compatibility between a super resolution optical disk which is provided with a masking layer and an ordinary optical disk provided with no masking layer in every one of the aforementioned techniques.